If anyone is wondering...

My code above does seem to test out working.
I modified my clock code to temporarily change the time to Sun March 13 01:59:45 (15 sec before DST should start) and then watched it change from 01:59 to 03:00 as it was supposed to.

Then I changed the time to Sun Nov 06 01:59:45 and watched it change from 01:59 to 01:00 as it should.

My nixie clock checks the date and time every morning at 05:00 and then calls my routine to test if STD/DST is in effect.
So over the next year it will be tested thoroughly.

In my case the nixie display is turned off between 10 pm and 6 am but it is still keeping time.

I'll have to ponder how to implement the above calculations into my overall code as I think I would still need to do a comparison between the current date and the calculated Start/End of DST.

I can't just abandon my baby (my version of the code) as I slaved over it for several hours
and it was a great challenge (as all code writing is)

But I will ponder implementing the apparently more simpler version in the near future.

thanks everyone